The Soft Wood Board can be found at the western side of the Fields, at one of the corners of a large empty field. Once you pick it up, though, a large Road Blocker will come and pass through that field. So you must be careful and get out of the Spirit's way as soon as it comes out upon you picking the wooden board up.
The Soft Wood Board is used to patch up the broken bridge of the Fields that leads deeper into the area. After you pick up the wooden board, you just need to place it on top of the gap between the sides of the broken bridge so that you can go across the bridge.